Mastiffs are huge dogs, that often outweigh their owners. They grow very fast as puppies and so their diet needs careful attention. Puppies should not eat puppy food past the first 2 to 4 months, because it encourages growth at too fast a rate. Your mastiff should visit the veterinarian when you first get it, and at regular intervals throughout its life, as recommended by your veterinarian. Feeding twice daily is recommended for all dogs, but especially important for mastiffs because they eat so much food each day and it is not good to fill up with a large amount all at one meal. Also, it keeps the dog from being hungry for half the day. It's OK to feed your mastiff 3 times daily if you prefer.
Read the label on the dog food. Check the daily feeding amount for your dog's age and size. If your dog is bigger than average or active, start with the highest of the range, and start in the middle or lower end of the range for a couch potato.
Divide the total amount recommended on the label by 2. The result is the amount to feed at each meal, morning and evening.
Measure your dog's food at each meal, rather than guessing. Feed half of the daily recommended amount in the morning and half in the evening.
Observe your dog's weight and adjust the amount of food as needed. You should be able to feel your dog's ribs if you press gently on its sides, but you should not be able to see the ribs. Your dog should have a "waist" just in front of the hip bones. If you see your dog's waistline increasing or can't feel his ribs, decrease the amount of food, still dividing the daily amount into two equal meals. Of course, if your dog is getting thinner and bony looking, increase the daily amount of food.
Keep fresh, clean water available at all times. Mastiffs need a lot of water, and they don't like water that is dirty or full of slobber.